1. Field of the Invention
The present invention relates to a rubber composition for a tire tread, and more particularly to a rubber composition providing a tire which exhibits improved abrasion resistance while the excellent performance on wet roads and the low fuel consumption exhibited by a tire using a conventional rubber composition containing aluminum hydroxide are maintained.
2. Description of the Related Arts
Carbon black is generally used as the reinforcing filler for rubber compositions because carbon black can provide higher reinforcing ability and more excellent abrasion resistance than other fillers to rubber compositions.
Energy saving is a recent social requirement and smaller heat buildup in a rubber composition for a tire, which means smaller rolling resistance of a tire, is required to achieve reduction of fuel consumption of automobiles. For this purpose, the amount of carbon black used in the rubber compositions may be decreased or carbon black having greater particle diameters may be used. However, it is known that the reinforcing property, abrasion resistance and the gripping property on wet roads are inevitably deteriorated in both cases.
As the filler which can satisfy the requirement for the low heat buildup while the reinforcing property, abrasion resistance and the gripping property on wet roads are maintained, precipitated silica is known, and many patent applications have been made on them. Examples of such applications include Japanese Patent Application Laid-Open Nos. Heisei 3(1991)-252431, Heisei 6(1994)-248116, Heisei 7(1995)-70369, Heisei 7(1995)-188466, Heisei 7(1995)-196850, Heisei 8(1996)-225684, Heisei 8(1996)-245838 and Heisei 8(1996)-337687.
However, precipitated silica has a drawback in that a rubber composition containing precipitated silica has a smaller storage modulus than a rubber composition containing carbon black having approximately the same specific surface area, and provides a tire showing inferior driving performance on dry roads.
It is known that the gripping property on wet roads can be improved by raising the glass transition temperature (Tg) of rubber, i.e., by increasing tan xcex4 at 0xc2x0 C. However, raising Tg of rubber causes problems in that properties at low temperatures become inferior, and in that rolling resistance increases, i.e., the low fuel consumption deteriorates.
Various technologies have been disclosed to overcome the above problems. Examples of such technologies include: (1) a rubber composition for a tire tread which provides an improved gripping property on wet roads by the use of a specific silica and of an improved mixing method (European Patent No. 501227); (2) a rubber composition for a tire tread which provides improved wet skid resistance while maintaining low heat buildup property without adverse effect on workability and abrasion resistance (Japanese Patent Application Laid-Open No. Heisei 7(1995)-149950); (3) a rubber composition for a tire tread which provides an improved gripping property on wet roads and semi-wet roads in low and high temperature ranges and improved workability (Japanese Patent Application Laid-Open No. Heisei 8(1996)-59893); and (4) a rubber composition for a tire tread which provides an improved gripping property on wet roads and semi-wet roads in low and high temperature ranges without adverse effect on abrasion resistance (Japanese Patent Application Laid-Open No. Heisei 8(1996)-59894).
However, the above technologies have drawbacks. In the technology described in (1), the rubber composition shows inferior workability (processability). In the technology described in (2), the rubber composition does not provide sufficient abrasion resistance. In the technologies described in (3) and (4), the reinforcing filler must be used in an excessively great amount.
On the other hand, it is known that aluminum hydroxide can be used as a reinforcing filler for rubber. A tire in which a rubber composition containing aluminum hydroxide is used for its tire tread shows excellent performance, such as the gripping property on wet roads, and provides the low fuel consumption. However, this tire has a drawback in that abrasion resistance is inferior.
An object of the present invention is to provide a rubber composition for a tire tread which, when the rubber composition is used for a tire tread for automobiles, provides improved abrasion resistance while the excellent performance on wet roads and the low fuel consumption exhibited by a tire using a conventional rubber composition containing aluminum hydroxide are maintained.
As the result of intensive studies by the present inventors to develop a rubber composition having the above advantageous properties, it was found that the above object can be achieved by mixing a specific amount of specific aluminum hydroxide particles with a rubber component selected from the group consisting of natural rubber and synthetic diene-based rubbers and using a specific amount of carbon black and/or silica powder in combination. The present invention has been completed on the basis of this knowledge.
The present invention provides a rubber composition for a tire tread which comprises (A) a rubber component selected from the group consisting of natural rubber and synthetic diene-based rubbers, (B) 5 to 50 parts by weight per 100 parts by weight of the rubber component of (b1) aluminum hydroxide particles which are treated with a surface treating agent on a surface and have an average diameter of secondary D2 of 10 xcexcm or smaller or (b2) aluminum hydroxide particles which have an average diameter of secondary particles D2 of 0.8 xcexcm or smaller and a ratio (D2/D1) of the average diameter of secondary particles D2 to an average diameter of primary particles D1 of 1.7 or smaller and (C) 5 to 80 parts by weight per 100 parts by weight of the rubber component of at least one filler selected from carbon black and silica.
In the rubber composition of the present invention, natural rubber and/or synthetic diene-based rubbers are used as component (A). Examples of the synthetic diene-based rubber include synthetic polyisoprene rubber (IR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), chloroprene rubber (CR) and butyl rubber (IIR).
A single type or a combination of two or more types of natural rubber and/or synthetic diene-based rubbers may be used as component (A).
As the aluminum hydroxide particles D2 of component (b1) used in the rubber composition of the present invention, aluminum hydroxide particles which are treated with a surface treating agent on the surface and have an average diameter of secondary particles D2 of 10 xcexcm or smaller are used. The surface of the aluminum hydroxide particles (aluminum hydroxide powder) is treated with a surface treating agent so that particles having particularly large diameters in the used particles do not work as nuclei of failure and formation of aggregates of aluminum hydroxide particles which may work as nuclei of failure is prevented.
When the average diameter of secondary particles D2 in the aluminum hydroxide particles which are treated with a surface treating agent on the surface exceeds 10 xcexcm, the reinforcing effect is not sufficiently exhibited to cause inferior abrasion resistance and, moreover, the gripping property on wet roads (the performance on wet roads) becomes inferior. Moreover, as the average diameter of secondary particles D2 becomes smaller, the particles agregates more readily. As a result, the properties of the rubber compostion may be detriorated due to the insufficient dispersion of the particles into rubber. From the standpoint of the balance between abrasion resistance, the performance on wet roads and the low fuel consumption, the average diameter of secondary particles D2 in the aluminum hydroxide particles is preferably in the range of 0.2 to 10.0 xcexcm and more preferably in the range of 0.4 to 0.8 xcexcm.
The surface treating agent used for the treatment of the surface of the aluminum hydroxide particles is not particularly limited and a desired agent can be selected from various conventional surface treating agents. Among the conventional surface treating agents, silane coupling agents and stearic acid are preferable and silane coupling agents are more preferable.
Examples of the silane coupling agent include compounds represented by the general formula Ra(RO)3xe2x88x92aSixe2x80x94A1xe2x80x94Smxe2x80x94A2xe2x80x94Si(OR)3xe2x88x92aRa or Xxe2x80x94Smxe2x80x94A1xe2x80x94SiRa(OR)3xe2x88x92a, wherein R represents a group which can be hydrolyzed, such as methyl group and ethyl group, X represents a functional group reactive with organic substances such as mercaptoalkyl groups, aminoalkyl groups, vinyl group, epoxy group, glycidoxyalkyl groups, benzothiazolyl group and N,N-dimethylcarbamoyl group, A1 and A2 each represents an alkylene group having 1 to 9 carbon atoms, m represents a positive number satisfying the relation: 0 less than mxe2x89xa69, and a represents a real number of 0 to 2. Specific examples of the silane coupling agent include sulfide silane compounds, such as bis(3-triethoxysilylpropyl) polysulfide, bis(3-trimethoxysilylpropyl) polysulfide, bis(3-methyldimethoxysilylpropyl) polysulfide, bis(3-triethoxy-silylethyl) polysulfide, 3-trimethoxysilylpropyl-N,N-dimethylcarbamoyl polysulfide, 3-trimethoxysilylpropylbenzothiazolyl polysulfide and 3-trimethoxysilylpropylmethacryloyl monosulfide; mercaptosilane compounds such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane and 3-mercaptopropylmethyldimethoxysilane; vinylsilane compounds, such as vinyltriethoxysilane and vinyltrimethoxysilane; amino compounds, such as 3-aminopropyltriethoxysilane and 3-aminopropyltrimethoxysilane; and glycidoxysilane compounds, such as xcex3-glycidoxypropyltrimethoxysilane and xcex3-glycidoxypropylmethyldiethoxysilane. Among these compounds, vinylsilane compounds are preferable.
As the aluminum hydroxide particles (aluminum hydroxide powder) of component (b2) in the rubber composition of the present invention, it is necessary that aluminum hydroxide particles which have an average diameter of secondary particles D2 of 0.8 xcexcm or smaller and a ratio (D2/D1) of the average diameter of secondary particles D2 to an average diameter of primary particles D1 of 1.7 or smaller be used.
The average diameter of secondary particles D2 is an average diameter obtained by the measurement using a laser diffraction type analyzer of distribution of particle diameters after dispersion of the particles by ultrasonic vibration. The average diameter of primary particles D1 is an average diameter obtained from the BET specific surface area in accordance with the following equation:
D1=6/{(BET specific surface area)xc3x97(true specific gravity)}
wherein the BET specific surface area is a value obtained in accordance with the one-point method of nitrogen adsorption of Japanese Industrial Standard R1626 after drying the sample at 110xc2x0 C. for 30 minutes.
When the aluminum hydroxide particles have an average diameter of secondary particles D2 exceeding 0.8 xcexcm or an excessively large average diameter of primary particles D1, the reinforcing effect is not sufficiently exhibited, which causes inferior abrasion resistance and, moreover, the gripping property on wet roads (the performance on wet roads) becomes inferior. When the particles are excessively small, aggregation of the particles becomes stronger and the ratio (D2/D1) of the average diameter of secondary particles D2 to the average diameter of primary particles D1 may exceed 2, then the particles cannot be dispersed sufficiently into rubber and a rubber composition having the desired properties cannot be obtained. From the standpoint of the balance among abrasion resistance, the performance on wet roads and the low fuel consumption, it is preferable that the aluminum hydroxide particles have an average diameter of secondary particles D2 of 0.8 xcexcm or smaller and more preferably 0.5 xcexcm or smaller, an average diameter of primary particles D1 of 0.35 xcexcm or smaller and more preferably 0.30 xcexcm or smaller, and a ratio D2/D1 of 1.7 or smaller and more preferably 1.5 or smaller.
In the present invention, a single type or a combination of two or more types of the aluminum hydroxide particles may be used as component (B). The content of component (B) is in the range of 5 to 50 parts by weight per 100 parts by weight of component (A). When the content is less than 5 parts by weight, sufficient gripping on wet roads cannot be obtained and the object of the present invention cannot be achieved. When the content exceeds 50 parts by weight, abrasion resistance deteriorates and there is the possibility that other physical properties required for a rubber composition are adversely affected. When abrasion resistance and the low fuel consumption are considered, the content of component (B) is preferably in the range of 10 to 30 parts by weight.
In the rubber composition of the present invention, at least one filler selected from carbon black and silica is used as component (C). Examples of the carbon black include channel black, furnace black, acetylene black and thermal black which are produced in accordance with different processes. Any of these types of carbon black may be used. Carbon black having a surface area by nitrogen adsorption (BET) of 90 m2/g or greater and a dibutyl phthalate absorption (DBP) of 100 ml/100 g or greater is preferably used. When BET is smaller than 90 m2/g, it is difficult that sufficient abrasion resistance is obtained. An excessively great BET causes deterioration in the low fuel consumption. When abrasion resistance and the low fuel consumption are considered, the more preferable range of BET is 90 to 300 m2/g. BET of carbon black is measured in accordance with the method of ASTM D3037-88. When DBP is smaller than 100 ml/100 g, it is difficult that sufficient abrasion resistance is obtained. An excessively great DBP causes deterioration in the low fuel consumption. When abrasion resistance and the low fuel consumption are considered, the more preferable range of DBP is 50 to 200 ml/100 g. DBP is measured in accordance with the method of Japanese Industrial Standard K6221-1982 (method A).
The type of silica used is not particularly limited. Silica can be suitably selected from those conventionally used for reinforcement of rubber composition, such as dry silica and precipitated silica. When abrasion resistance and the low fuel consumption are considered, silica having a specific surface area by nitrogen adsorption (BET) in the range of 70 to 300 m2/g is preferable. BET of the silica is measured in accordance with the method of ASTM D4820-93 after drying the sample at 300xc2x0 C. for 1 hour.
In the rubber composition of the present invention, when silica is used as component (C), the composition may further comprise a surface treating agent as component (D) to enhance the effect of component (C), where desired. The surface treating agent is not particularly limited. A surface treating agent can be suitably selected from various conventionally used surface treating agents. Among such surface treating agents, silane coupling agents are preferable. Examples of the silane coupling agent include the compounds described above as examples of the silane coupling agent for surface treating.
In the present invention, a single type or a combination of two or more types of the surface treating agent may be used where desired. In general, the content is selected in the range of 1 to 20% by weight of the amount of the silica in component (C). When the content is less than 1% by weight, there is the possibility that the effect of the surface treating agent is not sufficiently exhibited. When the content exceeds 20% by weight, the effect of the surface treating agent may not be exhibited to the extent expected from the used amount and, moreover, the amount may be economically disadvantageous. When the effect of the surface treating agent and economy are considered, the content of the surface treating agent is preferably in the range of 2 to 15% by weight of the amount of silica.
The rubber composition of the present invention may further comprise, where desired, various additives generally used in the rubber industry, such as vulcanizing agents, vulcanization accelerators, antioxidants, scorch retarders, softeners, other fillers, zinc oxide and stearic acid so long as the object of the present invention is not adversely affected.
The rubber composition of the present invention obtained as described above can be used for a tire tread. When the rubber composition is used for a tire tread, a tire having an excellent balance among the performance on wet roads, the low fuel consumption and abrasion resistance is provided.
To summarize the advantage of the present invention, the rubber composition of the present invention is used for a tire tread and provides a tire which exhibits improved abrasion resistance while the excellent performance on wet roads and the low fuel consumption exhibited by a tire using a conventional rubber composition containing aluminum hydroxide are maintained.